pearlym I think her pay/conditions are OK as they stand (esp 10 weeks holiday whether her choice or yours!! but you can't use that to pay the rent/bills/food), however, I also think that a 'truely wonderful' nanny is hard to find and something you might not even totally appreciate until you've been though 'childcare hell' in one way or another!! Mind you, as you want to treat her like Gold and think she's 'truely wonderful' you do seem to realise you have a good one
I think the things to consider here are 1. What is it worth to have her stay
- What can you afford.
Having her stay is not only nice, it's cost effective!
It's easy to say she's well paid/overpaid or has great perks etc if you aren't living in London! It's an expensive place to live (and near London, still isn't IN London).
I think we all like to feel appreciated and if things stay the same, you don't really feel appreciated/acknowledged do you... so if you can afford a raise then great, if not (as buggylovin said) a bonus or if money is quite tight, what about a nice treat (tickets to something she'd enjoy/vouchers for a meal out), just 'something' to say 'we think you're great!!'.
Re another DC, yes it does depend on the contract, but also expectations. Your nanny might be happy with her lot with one small child, but might not think it enough to have a toddler and a newborn. It doesn't matter what anyone else thinks, it's if it's not worth it, to her. I'd definitely sound her out about it sooner rather than later.